1. Field of the Invention
The present invention relates in general to electronic circuits as used within Supplemental Inflatable Restraint (SIR) systems for automotive vehicles and the like, and, more specifically, to semiconductor integrated driver circuits which are employed in such automobile airbag systems for deploying airbags using igniters or squibs; and including a firing control section guaranteeing secure operation under any circumstances and also including circuit diagnostics for testing the proper operation capabilities of the drivers that supply the electrical energy to ignite the squibs.
2. Description of the Prior Art
The main components of Supplemental Inflatable Restraint (SIR) systems for automotive vehicles and generally used as passenger protection systems in motor vehicles include an inflatable textile bag, named airbag, a propellant source (made e.g. from sodium azide pellets), an igniter or squib to initiate burning of the propellant source by means of a firing circuit driving said squib for causing inflation of the airbag. The squib is a detonator wire used for example to ignite the explosive charge that inflates the airbag in the automobile. The squib is a low resistance conductive wire with multiple coatings of an explosive material. At impact during an automobile accident, a large current flows through the squib, heats the wire, and ignites the explosive layers. This initial explosion sets off a secondary charge that inflates the airbag to protect the occupants. Alongside also necessary as another main component is a deployment circuit having at least one accelerometer as crash sensor, sensitive to vehicle motion, especially deceleration, and containing an electronic control module for determining when to deploy the airbag and sending a deployment pulse to the igniter, normally using a microprocessor system monitoring the accelerometer output for evaluating the severity of a crash to determine whether to deploy the airbag. The airbag, propellant, and igniter are by default contained in an airbag module (e.g., within a steering wheel for a driver airbag). The crash or safing sensor can be packaged separately or may be contained within the electronic control module.
This control module performs basic self-diagnostic monitoring of the SIR system each time the system is turned on (e.g., every time a vehicle is started). Any potential performance problems are identified and a warning light e.g. is illuminated on the console, so that the automobile driver knows that the system needs to be serviced. The vehicle battery voltage or ignition system voltage empowering these deployment and firing circuits will, by the way, subsequently be called Airbag Voltage Supply (AVS) voltage. The control circuit and the firing circuit are in general and to a large extent embodied with a microprocessor on an integrated circuit chip for the airbag deployment functionality, altogether called a squib driver. In any case an external harness leads to the squib at the site of the airbag, or in the case the vehicle is equipped with more airbags, this harness connects the squibs of each airbag to the squib driver chip. It is conceivable that eventually some portion of the harness might become damaged, i.e. short to ground (GND) or even to the AVS voltage. To forestall the possibility that such inadvertent electrical connection to the firing circuit might cause deployment of the airbag or disable the whole system, it is desirable to continuously monitor the system to detect any such event. It is common practice to perform diagnostic monitoring of the electrical connection of the squib elements, squib resistances, and electrical leakage or isolation in the squib circuits, among other tests. It is already known to diagnose a short of the firing circuit by feeding a small test current through the squib and through a biasing resistor to ground. The resulting bias voltage will depend on the current and if there are no shorts to the firing circuit the resulting test voltage will be at a prescribed value. To assure this, the test current has to be carefully controlled. Where, for example, if the current is provided by a current source on an integrated circuit, the circuit must be trimmed during manufacture to assure the correct current output. Then the test voltage is sampled by an A/D converter e.g. and fed to the microprocessor where it is monitored to detect a low value indicative of shorting to ground or a high value indicative of shorting to supply voltage. Thus the monitoring circuit in the automobile continuously checks the squib resistance and reports values outside the acceptable range as a warning.
Preferred prior art circuits obeying to automotive industry regulations do not allow a direct galvanic connection of the squib to the supply voltages—neither to the AVS voltage nor to GND voltage with possible firing current flowing—during normal operation of the vehicle, i.e. not firing the squib, and therefore include two separate switches on every side of a squib. This means one high-side switch connects the squib to AVS voltage and another low-side switch connects the squib to GND voltage, thus the deployment of a squib is only initiated if these two independent switches of the firing loop are both closed; the firing loop formed by the vehicle battery between AVS and GND on one side and with said high-side switch, said squib to be fired and said low-side switch on the other side. Currently said squib driver switches are implemented using NPN, NMOS or DMOS transistors, which are controlled as switches by appropriate driver circuits. Due to supply voltage variations and deviations from the normal squib resistance from about 2 Ohms the current through the switches may vary between approximately 2 A and 4.5 A. For integrated squib power transistor switches however the current flowing through these switching devices must be limited to approximately 3 A, therefore it has to be measured and limited during said firing operation and additionally the transistors must be forced to reduce their on-resistance to avoid excessive power dissipation, which could lead to a premature destruction of said semiconductor switching devices. The current practice of sensing this current with resistors causes excess voltage drop that in consequence enlarges the unwanted power dissipation within the integrated power circuit. This disadvantage poses a major problem for that sort of circuits.
Another consequence of the aforementioned automotive industry requirements is the measurement of the isolation of the squib against AVS and GND, and also the determination of the squib resistance, which has to be performed during normal operation of the vehicle. Normal operation means that the squib is not firing. These measuring operations should be periodically performed by applying repeatedly cycling resistance measurements during normal operations of the vehicle. In one aspect, prior art solutions provide a method of testing a high-side driver and a low-side driver in an airbag squib circuit. The airbag squib circuit includes a squib element coupled between the high-side driver and the low-side driver. The high-side driver controllably provides a high-side voltage to one side of the squib element and the low-side driver controllably provides a low-side voltage to the other side of the squib element. Resistance of the squib element is tested for a resistance value laying within a predetermined resistance range. Current leakage associated with said squib element is tested to determine whether it is over a leakage threshold. An intermediate voltage from a weak power supply is supplied to a test-point in the airbag squib circuit between the high-side driver and the low-side driver. One of the drivers is turned on while keeping the other one of the drivers off. Voltage at the test-point is continuously compared with a predetermined voltage range that includes the intermediate voltage. This one driver is turned off in response to the voltage at the test-point being outside the predetermined voltage range, thereby detecting that this one driver is operating properly. If the voltage at the test-point remains in the predetermined voltage range for a predetermined time period, then this one driver is turned off and an indication is made that this one driver has failed.
Unless there is a failure, the other driver is then turned on while keeping the one driver off. A voltage at the test-point is continuously compared with the predetermined voltage range. The other driver is turned off in response to the voltage at the test-point being outside the predetermined voltage range, thereby detecting that the other driver is operating properly. If the voltage at the test-point remains in the predetermined voltage range for the predetermined time period, then the other driver is turned off and an indication is made that the other driver has failed.
In daily use of these circuits in vehicles and especially under all the existing environmental conditions appearing on earth during operation of an automobile another major problem is the degradation of the devices during lifetime and therefore a multitude of techniques and methods for their implementations has been specified in the past.
The main problem hereby is due to the fact, that in most cases the known prior art solutions are very elaborate and complicated when all the requirements as established by the automotive industry have to be met, thereby resulting in practical realizations leading to very costly devices.
Realizations of the prior art for such circuits are inter alia implemented as specifically assembled semiconductor circuit systems, consisting of integrated control circuits combined with separate external switching devices or as already customized monolithic integrated circuits, which are however generally of rather high complexity and therefore high cost. FIG. 1A prior art partially shows a typical implementation of a system for airbag applications with VBat, the voltage of the car battery and the AVS supply voltage for the squib drivers, which is generated out of VBat with the help of a charge pump circuit. FIG. 1B prior art shows a typical realization of one of the four squib driver circuits from FIG. 1A prior art for explanatory purposes with only the squib driving parts and as exemplary prior art circuits for available integrated solutions.
These prior art circuits—which are in every case realized using two electronic switches, high-side and low-side switches connected to the squibs—have several drawbacks, which can especially be seen from the example in FIG. 1B prior art. The FET switches used herein are from a rather sophisticated structure, incorporating an especially enhanced drain, augmented by resistive components for the current measurement and current limitation during testing and firing operations. Furthermore an extra charge pump is needed in order to drive the high-side switch. For the limiting of the maximum driving current these additional resistors are used, which add a voltage drop thus enlarging the power consumption of the circuit. Another disadvantage is, that also for the current limitation a regulation loop is necessary, which can become instable. There are several such expensive solutions available with various patents referring to comparable approaches.
It is therefore a challenge for the designer of such devices and circuits to achieve a high-quality and also low-cost solution. Several prior art inventions referring to such solutions describe related methods, devices and circuits, as well as technologies.
U.S. Pat. No. 5,309,030 (to Schultz et al.) describes a current source for a supplemental inflatable restraint system, where a supplemental inflatable restraint system with a controllable current source comprises a sense element, a switch device coupled to the sense element, and a squib coupled to the switch device, wherein the sense element, switch device and squib are coupled in a series circuit between a power source and a ground. A deploy control circuit is coupled to the switch device, and, in response to a deploy command, forward biases the switch device allowing electric current to flow through the sense element, switch device and squib to deploy a supplemental inflatable restraint. A current control circuit is coupled to the sense element and switch device and responds to the current flow through the sense element and regulates the switch device, effectively limiting current through the series circuit to a predetermined value.
U.S. Pat. No. 5,506,509 (to Susak et al.) discloses a circuit and a method of measuring a squib resistance, where a resistance measuring circuit generates a predetermined reference voltage and impresses that reference voltage across a squib detonation device. The resulting current flowing through the squib is mirrored by a current mirror for providing multiple mirrored currents. The mirrored currents are compared to known current sources. The output signals go high or low depending on whether the mirrored currents are greater than or less than the fixed current sources. The output signals provide an indication as to whether the measured squib resistance is within a specified resistance range. The current sources may be precisely matched to maintain high accuracy in measuring the resistance.
U.S. patent application 2002/0050826 (to Boran et al.) shows and explains high and low side driver tests for airbag modules, wherein a method of testing a high-side driver and a low-side driver in an airbag squib circuit includes preliminary testing of squib resistance and squib leakage for a plurality of trials. Next, one of the drivers is turned on while keeping the other one of the drivers off. A current-limited power supply supplies an intermediate voltage to a squib terminal and the voltage at the terminal is continuously compared with a predetermined voltage range that includes the intermediate voltage. The one driver is turned off in response to the voltage at the point being outside the predetermined voltage range, thereby detecting that the one driver is operating properly. If the voltage at the point remains in the predetermined voltage range for a predetermined time period, then the one driver is turned off and an indication is made that the one driver has failed. If the first driver passed, then the other driver is tested in the same manner.
Although these papers describe circuits and/or methods close to the field of the invention they differ in essential features from the method and especially the circuit introduced here.